Optical fiber communications is seen as one of the most reliable telecommunication technologies to achieve consumers' needs for present and future applications. It is reliable in handling and transmitting data through hundreds of kilometers with an acceptable bit error rate and today, optical fiber communication dominate as the physical medium for medium and long distance data transmission systems and telecommunications networks. At the same time optical fiber solutions appear in short-haul applications, local area networks, fiber-to-the-home/curb/cabinet, and digital cable systems. Fundamentally, optical transmission systems are based on the principle that light can carry more information over longer distances in a glass medium than electrical signals can carry information over copper or coaxial cable.
Light is electromagnetic waves and optical fiber is a waveguide, and whilst very low loss in certain wavelength ranges, e.g. α<0.22 dB/km for Corning SMF-28 single mode silica fiber, ultimately in order to compensate the loss of the waveguide, an optical amplifier is needed. Doped fiber amplifiers (DFA) are optical amplifiers which uses rare-earth doping materials including, Erbium (Er3+), Praseodymium (Pr3+), Europium (Eu3+), Neodymium (Nd3+), Terbium (Te3+), Lutetium (Lu3+), Ytterbium (Yb3+), Holmium (Ho3+), Dysprosium (Dy3+), Gadolinium (Gd3+), Samarium (Sm3+), Promethium (Pm3+), Cerium (Ce3+), Lanthanum (La3+) and Thulium (Tm3+) inside the optical fiber. Essentially, within a transmission line the DFA is connected to a pump laser and works on principle of stimulated emission wherein the pump laser is used to provide energy and excite ions to an upper energy level. These excited ions are then stimulated by photons of the information signal and brought down to lower levels of energy such that they emit photon energy exactly on the same wavelength of the input signal. In addition to optical amplification for medium and long haul telecommunications, particularly within optical fiber communication systems (OFCS), DFAs are also employed, for example, as non-linear optical devices and optical switches.
In OFCS, the active medium of DFAs operating in the 1550 nm window is Erbium (Er3+) and significant research in the past 25 years has been addressed to their performance, optimization, and manufacturing resulting in thousands of publications on Erbium Doped Fiber Amplifiers (EDFAs) alone together with thousands of others to their use within systems and other optical elements of OFCS. Erbium doped silica based fibers which form the active medium within the EDFA are favoured as the emission of Er3+ ions is within a set of wavelength around 1550 nm where the silica fiber also exhibits the minimum attenuation on the information signal in its transmission via silica based fibers, such as Corning SMF-28 for example. EDFAs can gains as high as 40 dB, equivalent to 80 km of silica based singlemode fiber, with low noise. Important features of EDFAs include the ability to pump the devices at several different wavelengths, low coupling loss to the compatible fiber transmission medium and very low dependence of gain on light polarization due to the cylindrical shape of Erbium doped fiber. In addition, EDFAs are highly transparent to signal format and bit rate, since they exhibit slow gain dynamics, with carrier lifetimes of 0.1 to 10 ms, which result in the gain response of EDFAs being basically constant for signal modulations greater than a few kilohertz to tens of gigahertz. Consequently, they are immune from interference effects, such as crosstalk and inter-modulation distortion between different optical channels within a broad spectrum of wavelengths, typically a 30 nm spectral band referred to as the C-band ranging from 1530 to 1560 nm, that are injected simultaneously into the EDFA.
Subsequently, L-band EDFAs with flat optical gain from 1574 nm to 1604 nm and S-band EDFAs with gain from 1490 nm to 1520 nm, were established allowing dense wavelength division multiplexing (DWDM) at up to 160 channels, each operating at 10 Gb/s and with 50 GHz channel spacing. Whilst, there have been thousands of papers in the literature for optimizing gain, noise figure, gain flatness, etc as well as the design and integration of inter-stage elements such as dispersion compensation fibers (DCF) or gain equalization filters (GEF), are commonly located within the stages in order to solve the tradeoff between noise figure degradation, output power decrease, and inter symbol interference.
However, whilst research activities were focused to reducing noise figure and higher output powers, such as were achieved through combinations of increasing pump laser output power and multiple pump sources, one significant design parameter of the EDFA and in general DFAs received little emphasis and focus, this being the efficiency of the DFA in terms of the pump power converted into the output channel signal(s). This pump power conversion efficiency (PCE) became a focus when combined C+L band EDFAs were being developed as researchers exploited a variety of single, dual, multi-pump designs with single, double, triple and quadruple pass configurations such as discussed by Naji et al in “Review of Erbium-doped Fiber Amplifier” (Int. J. Phys. Sci., Vol. 6, pp. 4674-4689). However, here the primary focus was again increasing the L-band output power through these configurations as well as shifting the pumping wavelength from 980 nm or 1480 nm into the C-band, such as 1545 nm for example. In fact nearly twenty years after the first EDFA demonstrations fundamental analysis of PCE within erbium doped fiber (EDF) configurations began to define operating regimes and present alternatives to the prevalent use in high power applications of large mode area fiber with low numerical aperture (NA) to lower pump power intensity. Whilst this prevalent design approach reduces the nonlinear effects such as 980 nm pump excited state absorption it limits the power conversion efficiency at high power to approximately 30%.
This analysis, such as by Wang et al entitled “Novel Erbium Doped Fiber for High Power Applications” (Proc. SPIE Passive Components and Fiber-Based Devices 2005) showed that whilst PCE varies with pump power for constant NA and peak PCE occurs at different pump powers for different NA fibers it still only reaches 50-53%. Discrete PCE results have been reported above these values using titanium-sapphire lasers, such as Mahdi et al in “Single-Mode Pumping Scheme for EDFA with High-Power Conversion Efficiency using a 980 nm Ti:S Laser” (Microwave and Optical Technology Letters, Vol. 48, pp 71-74), where the PCE reached 60%, representing a quantum efficiency of 95%, these have been achieved using large research lasers and laboratory optical arrangements rather than the technician assembled semiconductor laser pumped configurations suitable for widespread deployment in telecommunications. Accordingly, the dominant commercial EDFA designs using large mode area fibers, production optical sub-assemblies, and commercial semiconductor laser diode pump sources operate at only approximately 30% power conversion efficiency from their pump signal, typically 980 nm, to the optical signals being amplified.
A high power EDFA operating at +23 dBm (200 mW) maximum output power requires approximately 600 mW of 980 nm pump power when operating at 30% efficiency. Within an EDFA module, such as for example a JDS Uniphase® WaveReady™ WRA-217 blade module, wherein the typical power consumption of the overall pump, coolers, control electronics etc is 18 W typically and 24 W maximum this “wasted” 400 mW of 980 nm optical power may not seem that significant. However, the power consumption of the 980 nm pump laser itself is approximately 2 W and the thermo-electric cooler (TEC) required to maintain the semiconductor laser diode operating temperature under varying ambient conditions typically consumes approximately between 2 W and 3 W at high ambient temperatures such as common within equipment cases and racks. The remaining power consumption is associated with network interfaces, power supplies etc which are only required where there are active electronic or electro-optical elements.
Accordingly, the 980 nm pump laser diode (LD) represents approximately 25% of the module power consumption directly, which is actually closer to approximately 40-50% once the control and drive electronics for the TEC and LD respectively are included within the calculation. It would therefore be beneficial to reduce the overall power consumption of a DFA by exploiting the unused optical pump power such that a lower power LD may be exploited thereby similarly reducing the requirements for TEC, TEC drive circuit, and LD drive circuit.
Accordingly, where multiple DFAs are to be employed in conjunction with one another such as for example at optical switches, optical cross-connects, and multi-channel reconfigurable optical add-drop multiplexers then every channel will exploit a similar DFA consuming, in the case of the WaveReady™ WRA-217, approximately 18 W. Accordingly, an 12×12 optical cross-connect, representing a cross-connect for example at the intersection of two links each comprising 6 optical fibers, 3 East and 3 West within a first ring and 3 North and 3 South in the second ring, with a DFA per channel therefore would consume 12×18 W=216 W of power. However, if the remaining optical pump power of the DFA can be re-used within another DFA then there is an opportunity to significantly reduce the power consumption of the DFAs associated with the optical cross-connect. For example, using the example above of DFAs consuming 200 mW maximum 980 nm pump power with a 600 mW 980 nm LD then potentially only a single LD may be employed to provide the optical pump power required across 3 DFAs. Accordingly, rather than the prior 12 DFAs with 12 pump LDs it would be beneficial to reuse the unused optical pump power such that 12 DFAs with only 4 pump LDs were required. Accordingly the DFAs would now consume only 4×18 W=72 W, representing a saving of 144 W.
It would be further evident, from the prior art analysis and experiments such as taught by Wang that the PCE of an optical amplifier varies with optical pump power such that for example a NA=0.14 EDF varies from a PCE of below 0.4 to above 0.52 as the pump power varies from about 50 mW to 300 mW. Accordingly, it would be beneficial to maintain a DFA within a predetermined operating regime for increased performance overall of the amplifier node from a power consumption viewpoint. It would also be evident that where multiple amplifiers are utilizing the same pump laser within a serial coupling of the pump to the multiple amplifiers rather than a parallel configuration that the power supplied sequentially between each pair of DFAs should be within a predetermined range in order to ensure that each amplifier operates as intended.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.